When a living organism attempts to perform a certain action governed by a specific part of a cerebrum, or receives a stimulus associated with the specific part of the cerebrum, neuron cells of the corresponding part of the cerebrum is activated. For example, when perception, calculation, judgment, or the like is carried out, a frontal lobe of the cerebrum and neuron cells corresponding to the frontal lobe are activated. At this time, in the peripheral blood vessels of the activated cells, the concentration of oxyhemoglobin is increased and that of deoxyhemoglobin is reduced in order to supply oxygen to the neuron cells. FIG. 1 shows an example in which the concentration of oxyhemoglobin is increased and that of deoxyhemoglobin is reduced as neuron cells are activated.
Near-infrared spectroscopy (NIRS) can measure the concentration changes of oxyhemoglobin and deoxyhemoglobin caused by the activation of neuronal cells. For example, a human body includes substances called chromophores, which have a chemical structure that can well absorb light of various kinds of specific wavelength bands. Hemoglobin is also a kind of the chromophores, and exhibits a larger degree of absorption than water in a near-infrared region. Since absorption coefficients of oxyhemoglobin and deoxyhemoglobin vary with wavelengths in the near-infrared region, information on the concentration changes of oxyhemoglobin and deoxyhemoglobin in the desired region can be obtained using light of two wavelengths in the near-infrared region.
FIG. 2 shows an example of absorption factors of oxyhemoglobin and deoxyhemoglobin, and FIG. 3 shows an example of a trajectory of light injected into a cerebrum.
First, FIG. 2 shows that the absorption coefficients of oxyhemoglobin (Oxy Hb) and deoxyhemoglobin (Deoxy Hb) are changed according to the wavelengths.
Further, FIG. 3 shows an example of a NIRS system by which light of a near-infrared region is emitted through a source (S) using a laser or a light emitting diode (LED) and the emitted laser or light is detected by a detector (D). Here, the light emitted by the source passes through specific parts of the brain along a curved path as shown in FIG. 3, and the NIRS system can obtain information on the parts of the brain through which the light has passed using information on the light detected by the detector. Background information on the NIRS system is well described in U.S. Patent Application Publication No. 2013/0256533.